The present invention relates to a process for the continuous monitoring of waste gas, (flue gas) from an incinerator plant, which contains volatile mercury halides and optionally metallic mercury in the form of vapor, with respect to their entire mercury content, and an apparatus to carry out the process.
Continuous monitoring is possible only when mercury occurs in its metallic (elemental) form.
A process for continuously monitoring such waste gases is known from DE-OS 37 04 533.
In the process disclosed in DE-OS 37 04 533, a continuous sample of gas stream is taken from the waste gas and guided through an activated charcoal bed at a temperature of approximately 350.degree. C. to reduce the volatile mercury halides.
In so doing, mercury halides, in particular HgCl.sub.2, are reduced on the activated charcoal into metallic mercury. The entire mercury content in the waste gas is then determined by measuring the metallic mercury content in the so pretreated sampling stream.
The drawback with this process is that impure substances, which prevent the reduction of the mercury or its release from the activated charcoal bed or reoxidize the already reduced mercury, can accumulate in the activated charcoal.
For this reason, the process can be used only with a so-called clean gas, in particular with hydrogen chloride concentrations below 100 mg/m.sup.3. Furthermore, the activated charcoal must be replaced from time to time.
Furthermore, it is known to guide the test gas (sample gas) stream, instead of through an activated charcoal bed, through a tin(II)-chloride solution or through a boron hydride solution, where mercury halides are also reduced to metallic mercury.
Such a process requires a continuous metering of reducing agents. In this process, the reductant content in the solution must be continuously monitored.
Furthermore, an alternative process can be derived from the aforementioned published DE-OS 37 04 533, in which process any and all kinds of reducing agents are dispensed with and in which the mercury halides are reduced at a temperature of over 700.degree. C. In so doing, existing hydrogen chloride must be set or reacted with calcium compounds. The high processing temperature makes the process time consuming.